Water molecules are very tiny, and they are also “polar” in nature, which means the molecules are not symmetrical: one end is bigger than the other, so they’re pointed, and the big end also has a “partial” positive electrical charge, while the small end is partially negative. They look a bit like a Mickey Mouse head in silhouette, with the ears being two hydrogen atoms. Their size, shape, and electrical polarity enable water molecules to be invasive and squeeze into the smallest spaces between the molecules of solids and pry them apart. This is dissolution, and the molecules, atoms, or small clusters that are liberated into the water (called a solution now) are said to be “dissolved” if they are too small ever to settle under the influence of gravity. Anything solid that is suspended in water but is large enough to settle eventually is a particle. Molecules and clusters of intermediate size are called colloids. Examples of colloidal materials include biological polymers such as protein molecules, tannins and lignins leached from vegetation, tiny aggregates of rust and clay, fragments of microbes and larger organisms, even complete viruses, asbestos fibers, etc., up to a size of a few tenths of a micron in diameter. This is also the same size range as the wavelengths of visible light (violet light is about 0.40 µm or 400 nm; red light, about 0.77 µm or 770 nm), and that is why particles of this size cause haziness or cloudiness in water, called turbidity.

Turbidity is measured by shining light through a test tube of the water and measuring the intensity of the light that comes out the other side. However, unlike ordinary photometers used in chemistry labs (which have the light source, sample, and detector all in a line), the detector for turbidity measurements is positioned at a 90° angle from the incoming light beam, so that only the reflected haze is measured. This is called nephelometry, and that is the origin of the Nephelometric Turbidity Unit, NTU. One NTU is not detectable by eye, but a reading of 15 NTU is noticeably cloudy. and murky river water may have a turbidity of several hundred. Public health or environmental regulations everywhere require the turbidity to be low at the time of disinfection, because pathogens can be shielded by particles. In the U.S., the requirement is <1 NTU 95% of the time, (<0.5 NTU for those systems using “conventional” or “direct” filtration), with an absolute maximum of 5 NTU. When this is exceeded, Boil Water Orders may be required by law. That is mostly because of protozoan cysts in surface waters, which are difficult or impossible to kill. The large urban waterworks using the most advanced water treatment methods are able to produce water with less than 0.5 NTU, but it invariably picks up millions of particles from the pipes and mains of the distribution system and may reach the point of use with several units of turbidity.

Discussion of Sizes and Dimensions: How big is 0.40 µm or 400 nm? The fundamental length or distance is the meter (symbol: m), but the unit most used in describing particles in water is the micro-meter (one-millionth of a meter) (symbol: um or µm), commonly called a micron. A meter is about a yard long; a millimeter (one-thousandth of a meter) (symbol: mm) is about 1/32 inch, and a micron is one-thousandth of that: 0.00003937 in. Also, many people think in terms of thousandths of an inch, or “mils” for very tiny measurements: a mil or 0.001 in. is 25.4 microns. A particle 40 microns In diameter is just barely visible to people with 20120 vision-smaller than 40 µm is microscope. The table below will help illustrate the range.

Comparison of Particle Sizes
1-inch ball

smallest item visible to naked eye
1 mil or 0.001 inch
fog droplet
silt and clay
pathogenic protozoan cysts
Cryptosporidium oocysts
Cyclospora cysts
Giardia cysts
Entamoeba cysts
red blood cells
most bacteria and algae
wavelengths of visible light

cigarette smoke
protein molecules
individual atoms

25.4 mm (millimeters)
=25,400 um or µm or “microns” (micro-meters)
10 – 100 microns
40 microns
25.4 microns
2 – 50 microns
40+ microns
1/2 – 20 microns
3 – 20 microns
3 – 7 microns
8 – 10 microns
8 – 12 microns
12 – 20 microns
7 1/2 microns
1/2 – 5 microns
0.1 – 5 microns
0.1 – 5 microns
0.40 (blue) – 0.77 (red) microns
= 400 (blue) – 770 (red) nm (nanometers)
10 – 1000 nm
10 – 250 nm
2 – 50 nm
0.05 – 0.25 nm
= 0.5 – 2.5 Angstroms
= 50 – 250 pm (picometers)